This invention relates to an improvement in a numerical control apparatus for controlling a tap threading operation.
FIG. 3 is a schematic diagram illustrating a device known in the art, capable of threading by a tap. In FIG. 3 reference numeral 1 designates a spindle; 2, a workpiece; 3, a cylindrical hole which is formed in the workpiece 2 in advance of the threading operation; 4, a cutter table; 5, a tap; 6, a floating tapper; and 7, an arrow.
In the threading operation, while the spindle 1 is rotated, the cutter table 4 is moved in a direction indicated by the arrow 7, so that the tap 5 is inserted into the hole 3 to thread the inner peripheral wall of the hole 3. To achieve highly precise tap-machining, synchronizing the rotation angle of the workpiece 2 is necessary; that is, the spindle 1 must be synchronized with the feeding amount of the cutter table 4 according to the thread pitch. Generally, however, the rotation control of the spindle 1 and the movement control of the cutter table 4 are performed independently by different servocontrol systems. Consequently, a floating tapper 6 has been proposed which allows the tap 5 to move freely in a direction perpendicular to the rotation direction thereof to compensate for a synchronization error between the rotation angle of the spindle 1, and the feeding amount of the tap 5 during tap-machining. More specifically, with such a floating tapper 6, even if the feed of the cutter table 4 is stopped while the spindle 1 is being rotated, the tap 5 is extended and retracted according to the spindle rotation angle. In other words, by using the floating tapper, the tap may be moved so as to meet the spindle rotation.
FIG. 4 is a block diagram showing the arrangement of a conventional numerical control apparatus. In FIG. 4, reference numeral 11 is a machining data memory section for storing machining data: 12 is an interpolation preprocess section for receiving the machining data and performing an arithmetical process prior to interpolation; 13 is an interpolation process section for performing an interpolation according to the output of the interpolation preprocess section 12; 14 is an acceleration and deceleration process section for performing acceleration and deceleration according to the output of the interpolation process section 13; 15 is a feed device for performing position control according to a position instruction provided by acceleration and deceleration process section 14; 16 is a speed instruction process section for outputting a speed instruction according to the output of interpolation preprocess section 12; 17 is a spindle device for performing speed control according to a speed instruction provided by the speed instruction process section 16; and 18 is an emergency stop process section for producing and inputting a stop instruction to the above-described process sections in response to an emergency stop signal.
The threading operation using, for example, a tap, is performed in the manner described below. First, machining data concerning the tapping operation are inputted to the interpolation preprocess section 12 by the machining data memory section 11. Then, the interpolation preprocess section 12 inputs predetermined speed instruction data to the speed instruction process section 16 according to the machining data. Instruction process section 16 converts the speed instruction data and direction of rotation data into voltages, or the like, which are then inputted to the spindle device 17. Spindle device 17 rotates the spindle in a predetermined direction according to the voltages corresponding to the speed instruction data, and to the direction of rotation data.
The interpolation preprocess section 12 applies machining data such as an amount of linear movement of the tap in a threading direction to the interpolation process section 13. Interpolation process section 13 performs an interpolation process according to the machining data, and inputs a position instruction to the acceleration and deceleration process section 14. The acceleration and deceleration process section 14 performs an acceleration, or deceleration, process for the position instruction, which is then inputted to the feed device 15. The feed device 15 controls the work table position, or the like, according to the position instruction.
An operation of removing the tap from the threaded hole, i.e., reversing the direction of spindle 17 rotation, and reversing the direction of feed device 15 movement, are performed similarly to the above-described threading operation.
Generally, when an emergency stop signal is inputted to the emergency stop process section 18 from either the feed device 15, the spindle device 17, or an operator upon an abnormal condition occurring during the threading machining operation, the respective process sections and the devices are placed in an emergency stop condition by original emergency stop signals such that the tap 5 is inserted into the hole 3 continuously. After recovery from the emergency condition, the tap 5 then is removed from the hole 3.
In the conventional numerical control apparatus thus constructed, the inertia of the spindle 1 is larger than that of the feed device 15, and the position control of the spindle device 17 is disregarded. Therefore, in removing the tap 5 from the hole 3, the positional error of the feed shaft, with respect to the angle of spindle rotation determined by the thread pitch, becomes very large, thereby resulting in a severe rearward movement that may deform the shape of the thread.
The above condition will be described in more detail hereinafter. FIG. 5 indicates the spindle rotation speed and the feed speed of the feed device in a tapping operation. As apparent in FIG. 5, after an instruction is issued to turn the spindle 17 in the forward direction, or in the reverse direction, at time instant a, the feed by the feed device 15 is started, and at time b, the feed speed reaches a value determined by a thread pitch and a spindle rotation speed. During the time period between times a and b, the feed speed is smaller than the determined value and the tapping operation cannot be performed; that is, the time period corresponds to an air-cut. At time c, in response to an emergency stop instruction, the spindle and the feed device start decelerating. The spindle speed is decreased according to the inertia and torque. At time instant e, the spindle 17 is stopped.
In this case, since the inertia of the spindle 17 is larger than that of the feed device 15, the time period between when the spindle device 17 starts decelerating until it stops is longer by a period of d to e than the time period between when the feed device 15 starts decelerating until it finally stops. Furthermore, no position control instruction is given to the spindle device 17, and, therefore, after receiving the stop instruction, the spindle rotation speed is reduced linearly whereas the feed device feed speed is reduced exponentially. Accordingly, for the time period between when the deceleration is started and when the spindle 17, or the feed device 15, is stopped, the ratio of the spindle rotation speed to the feed speed of the feed device 15 varies. As a result, a synchronization error may occur between the spindle rotation speed and the feed speed, thereby resulting in thread deformation.
The tap removing operation will be described hereinafter. At time instant f, an instruction for reversing the direction of spindle rotation and an instruction for moving the tap in a reverse direction are provided. For the time period between time instants f and h, similarly to the time period between time instants c and e, the ratio of the spindle rotation speed to the feed device feed speed in the reverse direction cannot be maintained at the value determined by the thread pitch. In other words, the synchronization between the spindle rotation speed and the feed speed in the reverse direction is destroyed. The time period from time instant h to time instant i involves removing the tap. That is, at time instant i, the tap has been removed from the threaded hole. During the time period between time instant i and time instant j, the feed device 15 decelerates and stops. At time instant j, the tap removing operation is completed. As described above, upon removing the tap from the threaded hole, the synchronization between the spindle rotation speed and the feed speed thereof may be destroyed, resulting in deformation of the threaded hole.
In the above-described process, a dummy mechanism such as a floating tapper is commonly used so that deformation of the thread portion is eliminated. In the dummy mechanism, a mechanism such as a splined shaft is employed so that an axis of the feed device for mounting the tap can be extended or retracted. Defects which may be caused by the inertial rotation of the spindle are absorbed by the dummy mechanism.
However, the conventional tap-machining process using the floating tapper is disadvantageous because the tap-machining is performed dependent upon the spindle movement and, thus, the accuracy in the thread portion length is poor. Furthermore, since there is a tap between the spindle and the feed device, the dummy mechanism normally can absorb the defects caused by the spindle's inertial rotation, but it cannot absorb the distortion due to the spindle's inertial rotation when the movable portion of the dummy mechanism is positioned at either its extended position limit or its retracted position limit, or due to the movement of an axis of the feed device when the spindle rotation stops. As a result, the bottom of the tapped hole is not accurate because the tap drifts in the bottom. Since preventing the amount of tap drift in the bottom from exceeding the range of play of the tap is critical, performing high-speed tapping operations is impossible.
In order to eliminate the above-described difficulties, Published Unexamined Japanese Patent Application No. 58-50454 has proposed a control system in which a pulse signal which is generated by the rotation of an encoder coupled to the spindle, is used as a z-axis feed pulse to synchronize thereby the spindle rotation with the feed amount in a z-axis. This eliminates the need for a floating tapper. However, in this case, eliminating the drift of the tap may be possible in a normal condition, but the control that is performed is dependent upon the spindle rotation, and therefore, accuracy is reduced in the thread portion length. Furthermore, removing the tap from operation when an abnormal condition occurs is somewhat difficult and time-consuming.
Accordingly, an object of this invention is to solve the above-described problems of the prior art. More specifically, an object of the invention is to provide a numerical control apparatus which is simple in construction and needs no expensive dummy mechanism such as a floating tapper, and which minimizes the drift of a tap in the bottom of a tapped hole. Thus, machining accuracy of the bottom of the tapped hole will be improved, and high-speed tapping operation may be conducted. Furthermore, according to the present invention, the numerical control device is capable of preventing the tap, and a threaded portion machined by the tap, from being damaged when an abnormal condition occurs.